The Olden Domain is said to be the best among the two in which Garm sang (i haven't listened to it extensively so I can't be certain), although i don't believe it can be described as progressive and I believe that The Archaic Course is the best of the albums in which Simen sang. You can get either of them they're both great.

I would recommend The Archaic Course, and it happens to be my fave Borknagar. I agree that Empiricism is too synth/key oriented, and TAC fulfills my need for the opposite quite well . To sum it up, it seems to be a straight ahead rocking metal album with folkish melodies/chord progressions, yet still maintains some elemnts of black metal. It is very guitar oriented, with synths coloring the music rather than leading it. The vocals are for lack of better description, unnatural! Simen comes up with some unexpected and absolutely chilling vocal melodies. If you can, check out Oceans Rise or Universal (first two track on the album), they should give you a decent feel for what the album is about.